The present application relates to a memory device including a memory layer storing a magnetization state of a ferromagnetic layer and a fixed-magnetization layer with a fixed magnetization direction, where the magnetization direction of the memory layer is changed by flow of an electric current. The present application also relates to a memory including such a memory device, which is suitably applied to a nonvolatile memory.
A high-density DRAM, which can be operated at high speed, has been used as a random access memory in information equipment, such as a computer.
However, the DRAM is a volatile memory from which information disappears when the power is turned off. Therefore, a nonvolatile memory, from which information does not disappear, have been demanded.
Furthermore, as a candidate of the nonvolatile memory, attention has been paid on a magnetic random access memory (MRAM) in which information is recorded based on magnetization of a magnetic material. Thus, the development of MRAM has been progressed.
MRAM allows electric current to pass through each of two different address wiring lines (a word line and a bit line) which are approximately perpendicular to each other to generate an electric field of electric current from each address wiring line. The current magnetic field inverts a magnetization of the magnetic layer of the magnetic memory device located at the intersection of address wiring lines to record information.
FIG. 1 shows a schematic diagram (perspective view) of a MRAM typically used in the related art.
A drain region 108, a source region 107, and a gate electrode 101, which constitute a selection transistor for selecting each memory cell, are formed on a portion separated by a device-separating layer 102 of a semiconductor substrate 110, such as a silicon substrate.
In addition, a word line 105 extends in the depth direction as shown in the drawing and is located above the gate electrode 101.
The drain region 108 is formed so that it can be used in common by selection transistors located on the right and left sides as shown in FIG. 1. The drain region 108 is connected to a wiring line 109.
Furthermore, a bit line 106 extends in the transverse direction as shown in the figure and is located above the word line 105. A magnetic memory device 103 having a memory layer, in which a magnetization direction is reversed, is arranged between the word line 105 and the bit line 106. The magnetic memory device may be, for example, formed of a magnetic tunnel junction device (MTJ device).
Furthermore, the magnetic memory device 103 is electrically connected to a source region 107 through a horizontally-extending bypass line 111 and a vertically-extending contact layer 104.
An electric current flows through each of the word line 105 and the bit line 106 to apply a current magnetic field to the magnetic memory device 103. As a result, the magnetization direction of the memory layer in the magnetic memory device 103 can be reversed, thereby recording information.
Furthermore, the magnetic layer (memory layer) for recording information may need to have a predetermined magnetic coercive force for stably retaining the information recorded in the magnetic memory, such as MRAM.
On the other hand, a certain level of an electric current should be passed through the address wiring line for rewriting the recorded information.
However, the address wiring line is thinly formed with the device in MRAM being miniaturized. Thus, a sufficient electric current is hardly passed through the address wiring line.
Attention has been focused on a memory, in which a magnetization direction is reversed with a spin injection, as one in which the magnetization direction can be reversed with a less level of an electric current (see, for example, Japanese Unexamined Patent Application Publication No. 2003-17782 (JP 2003-17782 A), U.S. Pat. No. 6,256,223, U.S. Patent Application Publication No. 2005-018439 A1, PHYs. Rev. B., 54.9353 (1996), and J. Magn. Mat., 159, L1 (1996)).
Reversing the magnetization direction with a spin injection is carried out such that a spin-polarized electron passed through a magnetic material is then injected into the other magnetic material to reverse the magnetization direction of the other magnetic material.
For example, an electric current is perpendicularly passed through the film surface of each of a giant magnetoresistive device (GMR device) and a magnetic tunnel junction device (MTJ device) to reverse the magnetization direction of at least part of the magnetic layer of these devices.
Furthermore, reversing the magnetization direction with a spin injection has an advantage in that the magnetization direction can be reversed without increasing an electric current even if the device is miniaturized.
Referring now to FIG. 2 and FIG. 3, the configuration of a memory in which a magnetization direction is reversed with a spin injection as described above will be described. FIG. 2 is a perspective view of the memory and FIG. 3 is a cross sectional view of the memory.
Device-separating layers 52 separate a semiconductor substrate 60, such as a silicon substrate, into parts. The separated part is provided with a selection transistor for selecting each memory cell. In other words, the selection transistor includes a drain region 58, a source region 57, and a gate electrode 51. The gate electrode 51 also functions as a word line extending in the depth direction as shown in FIG. 2.
The drain region 58 is formed so that it can be used in common by selection transistors located on the right and left sides as shown in FIG. 2. The drain region 58 is connected to a wiring line 59.
Furthermore, a memory device 53 having a memory layer with a magnetization direction to be reversed with a spin injection is arranged between the source region 57 and a bit line 56. Here, the bit line 56 extends in the traverse direction as shown in FIG. 2 and is arranged above the source region 57.
The memory device 53 is, for example, formed of a magnetic tunnel junction (MTJ) device. In the figure, the reference numerals 61 and 62 denote magnetic layers, respectively. One of the two magnetic layers 61, 62 is provided as a fixed-magnetization layer with a fixed magnetization direction and the other is provided as a free-magnetization layer on which the direction of magnetization can be changed.
In addition, the memory device 53 is connected to both the bit line 56 and the source region 57 through upper and lower contact layers 54, respectively. Therefore, the magnetization direction can be reversed with a spin injection by passing an electric current through the memory device 53.
Such a memory in which a magnetization direction is reversed with a spin injection has a simplified device structure in comparison with the typical MRAM illustrated in FIG. 1, so that the memory has an advantage of being integrated in high density.
In addition, when a magnetization direction is reversed with a spin injection, electric current for writing may not increase even the miniaturization of an element is progressed in comparison with the typical MRAM in which a magnetization direction is reversed with an external magnetic field.
In MRAM, writing lines (word lines and bit lines) for writing are formed in addition to a memory device so as to write (record) information by a current magnetic field generated by passing an electric current through each of the writing wiring lines. Thus, it is possible to feed a sufficient amount of electric current required for writing information into the writing line.
On the other hand, in the case of a memory in which a magnetization direction is reversed with a spin injection, the magnetization direction of a memory device may be reversed by the spin injection with an electric current passing through a memory device.
As described above, information can be written (recorded) by directly feeding an electric current to the memory device. Thus, a memory cell may be formed by connecting a memory device to a selection transistor so as to select the memory cell where the information is recorded. In this case, an electric current passing through the memory device is restricted to a certain level of electric current (saturated electric current of selection transistor) that is passed through the selection transistor.
Therefore, writing may need to be carried out with an electric current lower than the saturated electric current of the selection transistor. Accordingly, the efficiency of spin injection may need to be improved to lower the electric current passing through the memory device.
In order to increase the level of a read signal, it may be necessary to secure a high rate of magnetic resistance change. Thus, it is effective to provide a memory device with intermediate layers in contact with both sides of a memory layer as a tunnel insulating layer (tunnel barrier layer).
When the tunnel insulating layer is used as an intermediate layer as described above, the amount of an electric current passing through the memory device may be restricted to prevent the tunnel insulating layer from dielectric breakdown. From this point of view, it may be necessary to control the electric current in the spin injection.
The electric current level is proportional to the film thickness of the memory layer and the square of saturated magnetization of the memory layer. Thus, these factors (the film thickness and the saturation magnetization of the memory layer) may be adjusted in order to lower the electric current level (see, for example, F. J. Albert et al., Appl. Phy. Lett., 77,3809 (2000)). In addition, for example, Nguyen et al. discloses that the electric current level can be decreased by lowering the magnetization level (Ms) of a recording material (see, for example, U.S. Patent Application Publication No. 2005-018439 A1).
However, on the other hand, the memory may function when storing information written by such electric current. In other words, the stability of a memory layer to heat fluctuation (thermal stability) may need to be secured.
The memory device in which a magnetization direction is reversed with a spin injection has a small volume of the memory layer in comparison with the MRAM in the past. Thus, it can be considered that the thermal stability may be decreased.
If the thermal stability of the memory layer may not be secured, the reversed magnetization direction may be reversed again by heat and a writing error may thus occur.
Therefore, thermal stability is an important characteristic for the memory device using the magnetization reversal with a spin injection.
Therefore, for allowing the memory device in which a magnetization direction of a memory layer is reversed with a spin injection to act as a memory, an electric current required to reverse the magnetization with a spin injection may be reduced to the saturated electric current or less. In addition, it may be also required to ensure thermal stability for firmly retaining the written information.